This invention relates to a pipette head assembly of an automated assay instrument and, more particularly, to a method and device for calibrating a pipette head assembly, wherein the head assembly is adapted for use with disposable pipette tips.
Various types of chemical tests are performed by automated test equipment, an example of testing of considerable interest being the assay of biological substances for human health care. Automated test equipment allows large numbers of test samples to be processed rapidly. Such equipment is employed in health care institutions including hospitals and laboratories. Biological fluids, such as whole blood, plasma, or serum may be tested to find evidence of disease, and to monitor therapeutic drug levels, by way of example. In the automated test instrument samples of test fluids typically are provided in sample cups, and all of the process steps including pipetting of the sample onto an assay test element, incubation and readout of a test-result signal are performed automatically. The test instrument typically includes a series of work stations each of which performs a specific step in the test procedure. The assay element or cartridge is transported from one work station to the next by means of a conveyor, such as a carousel, to enable the test steps to be accomplished sequentially. The conveyor usually carries a plurality of the assay cartridges, each secured to a specific location on the upper surface of the conveyor. In the usual arrangement, the assay cartridges are spaced apart from each other in berths which are located along the periphery of the conveyor to facilitate automatic insertion and extraction.
In many automated instruments, a pipette head assembly is utilized in combination with disposable pipette tips which are used typically only once, and then discarded so as to eliminate a possible source of contamination of fluids, thereby to prevent errors in the assay results. In such systems, the pipette head assembly is required to pick up a disposable pipette tip, aspirate the appropriate fluid into the pipette tip, such as a sample fluid from a sample cup, and to dispense the required volume of fluid to the assay element. The fluid dispensing system in such instruments must meet various requirements which will be understood better from the following description of the typical manner in which a fluid dispensing assembly operates in an automated instrument.
In a typical construction of an automated instrument, the disposable pipette tips and the sample cups are arranged on carriers which are then placed on a carrier or tray supported by a movable table within the instrument. In one type of instrument, the pipette head assembly is transported horizontally (the X direction) and vertically (the Z direction), and the trays which hold the pipette tips and the sample cups are displaced from the front to the back of the instrument (the Y direction). The conveyor can be in the form of a carousel which carries the assay cartridges with rotational movement within an incubator. Initially, the pipette head assembly is driven downwardly to secure a disposable tip by frictional contact of a stem of the pipette head assembly with the interior of the disposable tip. Thereafter, a predetermined amount of fluid is aspirated into the pipette tip, and the tip is driven to a dispense position above an assay element where a predetermined volume of fluid is dispensed. Upon completion of the dispense step, the tip is discarded and a clean disposable tip secured to the stem of the pipette head assembly for the next dispense step.
The operation of the fluid dispense system is directed typically by software embedded in a microprocessor which controls the operation of the instrument. In order for the assays to be conducted properly, it is necessary that the pipette head assembly and, when present, the disposable tip secured thereto, be positioned accurately for aspiration of fluid and for expression of fluid. It is noted that aspiration of a small amount of fluid from a small plastic cup requires accurate positioning of the pipette tip to insure that no air bubbles are aspirated along with the liquid because of the resulting poor volumetric measurement. The sample cup carriers and the pipette tip carriers are usually formed as molded plastic parts, and their dimensions may vary slightly from one to another. Similarly, the assay elements typically have a molded polymeric housing, the dimensions of which may vary slightly from element to element. Also, in order to obtain a uniformly dispensed sample of liquid within the well of a sample cartridge, it is necessary that the pipette tip orifice be at a precisely controlled location above a central portion of the well of the cartridge.
The foregoing requirements of precision and accuracy in the positioning of the pipette can be accomplished only by a proper calibration of the test equipment to account for any minor variations which may be present in the attitude of a sample tray, and any minor deviations from expected position of the various components of the test equipment. Therefore, for automated assay instruments, it is highly desirable to have a technique for calibrating the fluid dispense system to minimize errors in the assay results which would otherwise occur because of dimensional variations in the various component parts and assay elements.